XRT72L56 Datasheet, PDF (307/486 Pages) Exar Corporation – SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

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PRELIMINARY
XRT72L56
SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
REV. P1.1.2
5.2.4 The Transmit E3 Framer Block
5.2.4.1 Brief Description of the Transmit E3
Framer
The Transmit E3 Framer block accepts data from any
of the following four sources, and uses it to form the
E3 data stream.
• The Transmit Payload Data Input block
• The Transmit Overhead Data Input block
• The Transmit HDLC Controller block
• The Internal Overhead Data Generator
The manner in how the Transmit E3 Framer block
handles data from each of these sources is described
below.
Handling of data from the Transmit Payload Data
Input Interface
For E3 applications, all data that is input to the Trans-
mit Payload Data Input Interface will be inserted into
the payload bit positions within the outbound E3
frames.
Handling of data from the Internal Overhead Bit
Generator
By default, the Transmit E3 Framer block will internal-
ly generate the overhead bytes. However, if the Ter-
minal Equipment inserts its own values for the over-
head bits or bytes (via the Transmit Overhead Data
Input Interface) or if the user enables and employs
the Transmit E3 HDLC Controller block, then these in-
ternally generated overhead bytes will be overwritten.
Handling of data from the Transmit Overhead Da-
ta Input Interface
For E3 applications, the Transmit E3 Framer block au-
tomatically generates and inserts the framing align-
ment bytes (e.g., the 10 bit FAS framing alignment
signal) into the outbound E3 frames. Hence, the
Transmit E3 Framer block will not accept data from
the Transmit OH Data Input Interface block for the
FAS signal.
However, the Transmit E3 Framer block will accept
(and insert) data from the Transmit Overhead Data In-
put Interface for both the “A” and “N” bit-fields.
If the user's local Data Link Equipment activates the
Transmit Overhead Data Input Interface block and
writes data into this interface for these bits or bytes,
then the Transmit E3 Framer block will insert this data
into the appropriate overhead bit/byte-fields, within
the outbound E3 frames.
Handling of data from the Transmit HDLC Control-
ler Block
The exact manner in how the Transmit E3 Framer
handles data from the Transmit HDLC Controller
block depends upon whether the Transmit HDLC
Controller is activated or not. If the Transmit DS3
HDLC Controller block is not activated, then the
Transmit E3 Framer block will insert a “1” into each
“N” bit-field, within each outbound E3 frame.
If the Transmit E3 HDLC Controller block is activated,
then data will be inserted into the “N” bit-fields as de-
scribed in Section 4.2.3.
5.2.4.2 Detailed Functional Description of the
Transmit E3 Framer Block
The Transmit E3 Framer receives data from the fol-
lowing three sources and combines them together to
form the E3 data stream.
• The Transmit Payload Data Input Interface block.
• The Transmit Overhead Data Input Interface block
• The Transmit HDLC Controller block.
• The Internal Overhead Data Generator.
Afterwards, this E3 data stream will be routed to the
Transmit E3 LIU Interface block, for further process-
ing.
Figure 119 presents a simple illustration of the Trans-
mit E3 Framer block, along with the associated paths
to the other functional blocks within the chip.
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